Spectral measurements of objects are required by various industries for different applications. The term ‘object’ herein refers to continuous printed substrates or discrete objects such as packages, fruits, vegetables, plastic toys, boxes and the like. For example, in the food industry, spectral measurements are used for sorting and quality control. In the biological materials industry, spectral measurements are used for the examination of dye markers absorbed by various cells or organisms, or to non-invasively identify pigments in a single cell or clusters of cells and to map the spatial organization of phototrophic groups in complex microbial communities. In the chemical industry, spectral measurements are used, for example, to identify the presence of pollutants in the air, by distinguishing the pollutants from the nitrogen, oxygen, water and other expected constituents.
In particular, spectral measurements are used in the printing industry for determining chromatic characteristics of objects. The chromatic characteristics may be represented, for example, by the estimated coordinates of a printed color in a color space (e.g., CIEXYZ, CIELUV, CIELAB, RGB, CYMK, HIS and the like), determined according to measurements of the light, which is either reflected from or transmitted through the object. The chromatic characteristics may further relate to the optical reflection densities of the inks deposited on a printed substrate or an object. The chromatic characteristics may also relate to the spectral composition of the light reflected from the printed substrate.
Objects, which at least a portion thereof, transmit at least a portion of incident light thereon, are herein referred to collectively as non-opaque objects. The term ‘non-opaque’ is defined, for example, in accordance with the ISO 13655 standard, where “opaque” is defined as a material with opacity greater than 99. A material behind the non-opaque objects may affect the reflectance factor of light impinging on the non-opaque objects. Alternatively, the material behind the non-opaque objects may affect the transmittance factor of light illuminating the non-opaque objects. When spectral measurements are made, the material behind the non-opaque object is referred to herein as a ‘backing.’ The reflectance factor can be defined as the ratio of light reflected from the object to light reflected from a perfect reflecting diffuser, both illuminated with the same light from the same direction. Similarly, the transmittance factor can be defined as the ratio of light transmitted through the object to light transmitted through a perfect transmitting diffuser, both illuminated with the same light from the same direction. The reflectance or transmittance factors may in turn affect the estimation of chromatic characteristics of the non-opaque objects, which are determined according thereto. For example, the reflectance factor is used when determining the X, Y and Z tristimulus values (e.g., according to the CIE 1931 XYZ color space) according to spectral measurements of the reflected light from the non-opaque object. Thus, for example, the determined X, Y and Z tristimulus values of a green color printed on a non-opaque material with a shiny aluminum backing will be different than the determined X, Y and Z tristimulus values of the same green color printed on the same non-opaque material with a white material backing.
To achieve repeatability and reproducibility of measurements of chromatic characteristics from a non-opaque object, as well as to provide a basis for comparing measurements between different measurement systems, a standard backing may be placed behind the non-opaque object during spectral measurements. During such spectral measurements, the printed material may need to be in proximate contact with the backing. The term ‘repeatability of measurement’ relates herein to a measurement of a sample, which can be repeated (i.e., for that same sample), with the same measuring instrument, under substantially the same measurement conditions, with substantially the same results (i.e., within a determined tolerance). The ‘reproducibility of measurement’ relates herein to a measurement of a sample, performed by an observer, with a measurement instrument, under specified measurement conditions, which can be reproduced by another observer, with substantially the same measurement instrument, under substantially the same measurement conditions, with substantially the same results (i.e., within a determined tolerance). For example, according to the ISO 13655 standard, the backing may be black, referred to as a black backing, or white, referred to as a white backing. Specifically, Annex A of the ISO 13655 standard defines the requirements of the white and black backings. The ISO 13655 standard further recommends that the measurements of light reflected off the non-opaque object, placed on and in contact with the backing, should be corrected relative to the reflection measurements of the backing alone. Similarly, other standards and industry accepted specifications such as those provided by the American Society for Testing and Materials (ASTM), the American National Standards Institute (ANSI), the Deutsches Institut für Normung (DIN), Fogra, and the Flexographic Technical Association (FTA), have various existing and evolving standards and specifications for colorimetric measurements. Such standards and specifications are, for example, FIRST, SNAP, Specifications Web Offset Publications (SWOP), General Requirements for Applications in Commercial Offset Lithography (GRACOL) and the like.
Systems and methods for the measurement of chromatic characteristics of non-opaque objects are known in the art. U.S. Pat. No. 5,047,652 issued to Lisnyansky, et al., and entitled “System for On-line Measurement of Color, Opacity and Reflectance of a Translucent Moving Web”, is directed to a system for on-line optical measurement of properties of a translucent moving web. The translucent moving web is conveyed on a backing roll. The backing roll has a surface which includes at least two portions respectively comprising the “white” and “black” optical standards. For example, one semi-cylindrical half of the backing roll surface comprises the “white” optical standard, and the other semi-cylindrical half of the roll surface comprises the “black” optical standard. The backing roll is positioned such that a circumferential portion of the roll surface contacts the back web surface where the web characteristic is to be optically measured. For measuring web characteristics, an optical sensing device is positioned adjacent the circumferential portion of the roll surface so as to view the front web surface backed by the one or more optical standards of the surface. The optical sensing device includes a scanning mounting operable to traverse in a direction parallel to the roll axis such that measurements can be made across the width of the web.
U.S. Pat. No. 5,818,497 issued to Kerr, et al., and entitled “Apparatus for Magnetically Coupling a Lead Screw to a Print Head”, is directed to an apparatus for coupling a lead screw to a print head. The print head is mounted on a movable translation stage member which, is supported for low friction slidable movement on translation bearing rods. The movable translation stage is magnetically coupled with a drive nut of a lead screw by a magnetic assembly. The lead screw includes an elongated, threaded shaft which is attached to a linear drive motor on its drive end. It is noted that rotation of the drive nut typically produces undesirable movement of the translation stage member. The magnetic assembly functions to provide a rotational constraint to the drive nut for consequently preventing the undesirable movement.
US Patent Application Publication No. 2009/0256087 to Engler Hans, et al., entitled “Measuring Apparatus Having a Movable Measuring Device in a Press” is directed to a measuring apparatus for scanning moving sheet-like printing materials. The scanned printed sheets are transported through a sheet transport section by transport cylinders. A measuring device is disposed above the sheet transport section. The measuring device has a movable runner, which carries the measuring optics. The linear drive moves the measuring optics concomitantly in parallel in the sheet transport direction during the measuring operation, so that the time of the measuring operation can be prolonged. During the measuring operation, the printed sheet can additionally be supported by a tray-like measuring support, in order to keep the distance between measuring device and printed sheet constant. The measuring support is moved concomitantly with the sheet and is additionally designed to be vertically adjustable by means of an adjusting element, so that the distance between printed sheet and measuring device can be regulated.
U.S. Pat. No. 4,003,660 to Christie, Jr., et al., entitled “Sensing Head Assembly for Multi-Color Printing Press On-line Densitometer” is directed to a sensing head assembly for multi-color printing press online densitometer. The sensing head assembly is moved across the width of the scanned web by a reversible electric motor driving a reversible rotating drive screw.
US Patent Application Publication No. 2004/0042022 to Friedman, Michael et al., entitled “Active Color Control for a Printing Press” is directed to a system for the accurate measurement and control of image color density on an operating printing press. The color control system scans a conveyed web. The color control system includes an imaging assembly including a CCD digital camera and a strobe light. The imaging assembly scans the top side of the web. Another imaging assembly including another CCD digital camera and another strobe light scans the bottom side of the web. Each imaging assembly is mounted on a respective carriage, which moves and positions the camera and strobe at an operator specified location across the web width.
European Patent No. EP 1 068 958, issued to Kerr, Roger Stanley and entitled “Method and apparatus for positioning a writing assembly of an image processing apparatus” is directed to an image processing apparatus including an imaging drum, and a print head. The print head is driven by a lead screw and stepper motor for moving along a line parallel to a longitudinal axis of the imaging drum as the imaging drum rotates.